The invention pertains to a hard composite that is made via sintering techniques. More specifically, the invention pertains to a hard composite that is made via sintering techniques wherein there are two distinct microstructural zones having complementary properties.
In hard composites like cemented tungsten carbides, the grain size, as well as the binder (e.g., cobalt) content each has an influence on the performance of the composite. For example, a smaller or finer grain size of the tungsten carbide results in a stronger and more wear resistant material. An increase in cobalt content typically leads to an increase in toughness. Thus, for certain applications there has been the desire to have a cemented carbide body that exhibits a finer grain size and desirable binder levels.
Heretofore, persons have been able to produce a hard composite having a fine grain size through the incorporation of grain refiners in the initial powder blend. This hard composite has a fine grain size throughout its microstructure. Persons have been able to make a hard body with a coarse grain size via sintering without the incorporation of any grain refiners since the tendency of a hard composite like a WC-Co composite is for the WC grains to coarsen during sintering. This hard composite has a coarse grain size throughout its microstructure. As can be appreciated these hard bodies have a uniform microstructure throughout and do not present a dual zone microstructure.
Persons have tried to produce a hard composite having two distinct microstructural zones. For example, Japanese Disclosure No. 52-110209 discloses two basic processes for making a cemented carbide product with two distinct zones. In one process, a green compact of 80 weight percent WC, 10 weight percent TiC and 10 weight percent Co was spray-coated with a slurry of 90 weight percent WC and 10 weight percent Co. After the coating dried, the substrate (and layer) was sintered, and then coated. In another process, a green compact of 94 weight percent WC and 6 weight percent Co was covered with a layer of 90 weight percent WC/10 weight percent Co powder. The compact was sintered, and then coated.
European Patent No. 194,018 shows the orientation of a cemented carbide part with a coarse-grained interior and a finer-grained exterior wherein the principal focus of the '018 European Patent is on a wire drawing die. In the manufacture of a wire drawing die, a large diameter mandrel helps form the geometry of the outer finer-grained zone, and the outer zone is pre-pressed. A small diameter mandrel helps form the geometry of the inner coarse grained zone. The entire compact is then sintered.
European Patent No. 257,869 discloses a cutting element made according to the following steps: (1) mixing a crown mixture of tungsten carbide powder and cobalt powder, with the cobalt powder being in the range of four to eleven percent (preferably nine to eleven percent) of the crown mixture; (2) mixing a core mixture of tungsten carbide powder and cobalt powder, with the cobalt powder being in the range of about twelve to seventeen percent (preferably fifteen to seventeen percent) of the core mixture; (3) providing a die having a cavity approximately the shape of the cutting element to be formed; (4) positioning in the cavity a quantity of the crown mixture in the shape of a crown defining at least the majority of the outer surface for the tip portion of the cutting element using a pressure of less than about 600 pounds per square inch; (5) positioning in the cavity a quantity of the core mixture sufficient to form almost all of the base portion and at least an inner part of the tip portion of the cutting element; (6) pressing the two quantities of the crown and core mixtures together and into the die at pressures in the range of about ten to fifteen tons per square inch; and (7) sintering the pressed insert (e.g., for about sixty minutes at about fourteen hundred degrees Centigrade) to form the cutting element.
None of these earlier documents shows a method of making a hard component with a dual zone microstructure wherein a powder is placed in contact with the surface of a green compact prior to sintering. This powder is sacrificial in that it does not form a microstructural zone. This powder also acts to influence the microstructure of the green compact during sintering.
Typical applications that would find hard composites with a dual zone microstructure useful, i.e., a peripheral zone of a finer grain size and an interior zone of a coarser grain size, are mining applications, construction applications, wear applications, and metalcutting applications. In the mining applications, mining tools like roof bits, open face style tools, and conical style tools would find a use for a hard insert with the dual zone microstructure. In the construction application, rotatable construction tools would find a hard insert with a dual zone microstructure to be advantageous. Wear parts like wire drawing dies would also find a hard component with a dual zone microstructure to be advantageous. In metalcutting applications, a cutting tool that has a dual zone microstructure would be advantageous.